1. Field of the Invention
This invention relates generally to an apparatus that compensates for drift in a perceived position of a jack leg.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Electro-mechanical jack systems are used in a wide variety of commercial and industrial applications. Because they generally comprise simple linear actuators, such jack systems can be integrated into mechanical systems for use in aligning or moving structures into desired positions relative to other structures or to applied forces or accelerations. One well known application of electro-mechanical jack technology is to control the attitude or tilt of a rigid or semi-rigid platform, such as a recreational vehicle, relative to earth's gravity.
Position sensing systems, such as optical encoders and Hall effect sensors are also known to be useful in many commercial and industrial applications. A typical optical encoder includes a rigid, opaque encoder ring that may be supported on a shaft or other structure whose rotation is to be monitored. The encoder ring may have angularly-spaced holes or slots that allow light to pass through. With a light source, such as an LED photoemitter, positioned on one side of the encoder ring, and a light detector, such as a phototransistor photodetector, positioned on the other side, a detection circuit connected to and receiving signals from the photodetector can sense whenever encoder ring rotation causes a hole/slot pass between the photoemitter and the photodetector. Because the holes/slots are spaced at regular angular intervals, rotation of the encoder ring will cause the photodetector to sense a continuous series of light pulses or pulse train as the shaft is spinning. By counting the pulses in a pulse train, the detection circuit can directly track the rotation of the encoder ring and shaft and can infer the motion of other connected structures. Additionally, the frequency/period of the pulse train can be used to calculate shaft rotational speed.
Hall effect sensors are typically mounted in fixed locations circumferentially spaced from one another and radially equidistant from a rotor magnet in positions allowing them to track rotor magnet rotation by sensing the passage of magnetic poles of the rotor magnet. Alternatively, a single Hall effect sensor may be mounted adjacent the circular path of an array of magnets circumferentially spaced around and supported on a rotatable disk or wheel. According to this arrangement the stationary Hall effect sensor tracks disk rotation by sensing the passage of the magnets.
When a position sensor such as an optical encoder or Hall effect sensor is employed, the position of a rotating structure such as a wheel or shaft can be accurately ascertained, and the position of a connected structure inferred, by counting pulses in the direction of rotation. An initial starting position of the rotating structure may be defined as being the pulse count of a pulse counter when the structure is in an initial starting or home position—typically or conventionally the pulse count at such a home position would be assigned a value of zero. Whenever a pulse is subsequently received it is then sensed whether rotation is clockwise or counter-clockwise. If clockwise, the pulse counter is conventionally incremented. If sensed rotation is counter-clockwise, the pulse counter is conventionally decremented.
Each pulse counted corresponds to a predetermined unit change in angular or rotational position of the shaft, as represented by the equationShaftRotation=KShaftRotationAnglePerPulse×PulsesCounted.
Where a jack includes a gearbox or other mechanical system that translates shaft rotation into linear extension/retraction of a leg portion of the jack, the position or degree of extension of the jack leg may be precisely calculated by the following equation:JackTranslation=KTranslationPerrotation×ShaftRotation JackPosition=InitialStartingPosition+JackTranslation =InitialStartingPosition+(KTranslationPerrotation.×ShaftRotation)
Accordingly, the total number of pulses counted is directly proportional to the distance the jack leg has traveled. The total number of pulses counted represents a position change or translation delta from a jack leg starting position that is unknown to the position sensor until the position sensor is “taught” what to consider as being the jack leg starting position.
The quality of sensor signals received from positions sensors such as Hall Effect sensors and optical position sensors can vary and is often not 100% accurate. For example, factors such as electrical noise, wear in mechanical elements of a drive system can periodically cause the loss of a pulse, or can cause additional pulses to be registered by a software counter. Each additional or missing pulse causes a small error in the calculated or “perceived” present position of a jack leg, i.e., the jack leg position perceived by a controller, resulting in a gradual drift of this perceived jack position away from the true or “actual” jack position. Although each missing or added pulse represents only a fraction of an inch of jack leg travel, over time, the errors will accumulate in either the extension or retraction direction and may eventually impair normal operation by, for example, reducing the controller's perception of the amount of jack stroke available.
Because such errors are generally random, they are impossible to predict and counter in a precise way. Consequently, drifting will occur. It would, therefore, be desirable for a jack or actuator system to be able to compensate for such errors and insure that, over time, the drift in perceived jack position is limited as much as is practical. It would also be desirable to automatically compensate for drift in perceived jack position without requiring user intervention.
It is also known for electro-mechanical actuators such as jacks to use software-defined soft stops. A soft stop is a position where a controller automatically stops the actuator, short of full extension or retraction to a stroke limit, to prevent the actuator from reaching such a stroke limit. Consistently stopping short of a stroke limit extends the life of an actuator by preventing the wear and tear that would otherwise result from repeated encounters with a hard physical stop defining a stroke limit.
It would be desirable to automatically limit the extent to which the perceived position of a leveling jack is allowed to drift in a direction that reduces the amount of stroke that a jack has available for leveling a platform or otherwise adjusting the position of an associated structure.